1. Field of the Invention
The present invention relates to an electric power steering apparatus for a vehicle, and more particularly, to an electric power steering apparatus for a vehicle which has an enhanced supporting structure for improving the bending strength of a rack bar.
2. Description of the Related Art
Typically, power steering apparatuses are divided into hydraulic power steering apparatuses using a hydraulic power and electric power steering apparatuses using an electric power based on the type of a power source.
Particularly, there are various kinds of electric power steering based on the installation position of a motor supplying a power. Among them, the apparatus driving a rack bar by a motor is a rack assist type electric power steering apparatus (R-EPS).
FIGS. 1 and 2 are cross sectional views showing a conventional rack assist type electric power steering apparatus. The structure thereof will be explained as follows.
That is, a rack bar 2 is contained in a rack housing 1. At the interior of one side of the rack housing 1 is installed a motor 3 with a stator 3a, a rotor 3b and a motor shaft 3c. 
At the interior of the motor shaft 3c is fixed a ball nut 4. On an outer surface of the rack bar 2 disposed inside of the ball nut 4 is formed a ball screw 2b engaged with the ball nut 4.
Meanwhile, at the other side of the rack housing 1 spaced from the installation position of the ball nut 4 is installed a pinion housing 5. Inside of the pinion housing 5 is installed a pinion shaft meshing with the rack bar 2.
On an outer surface of the pinion shaft 7 and the rack bar 2 are formed a pinion gear 7a and a rack gear 2a respectively. At the backward portion of the rack gear 2a are installed a rack guide 8 and a spring 8a. 
However, such a conventional electric power steering apparatus for a vehicle has a two-point support structure where both ends of the rack bar are supported by the ball nut and the rack guide. Therefore, it has a low bending strength and thus a large bending amount. This causes a rack gear portion, which is a relatively weak, to be easily damaged.
Therefore, it is an object of the present invention to provide an electric power steering apparatus for a vehicle which has a supporting bush installed in the position adjacent to a rack bar in order to improve the bending strength of a rack gear formed at the rack bar.
To achieve the above object, there is provided an electric power steering apparatus for a vehicle in accordance with the present invention, which comprises: a motor having a rotor, a stator and a motor shaft inside the same; a housing installed such that it can communicate with the motor; a rack bar installed inside of the motor and housing and having a ball screw installed at the outer circumferential surface of one end and a rack gear formed at the outer circumferential surface of the other end; a ball nut installed at the outside of the ball screw for supporting the rack bar at a first point and moving the same in the axial direction; and a rack guide installed at the inside of the housing for supporting the rack bar at a second point and guiding axial movement by enclosing the rear surface of the rack gear, where the rack bar is supported at a third point by a supporting bush disposed at the inside of the front end of the housing adjacent to the rack guide
In a preferred embodiment of the present invention, at the inner circumferential surface of the supporting bush, a clearance recess for separating the edge of the rack gear from the inner circumferential surface of the supporting bush may be formed, in order to prevent abrasion caused by the friction between the supporting bush and the rack gear.
Preferably, the clearance recess may be formed along the circumferential direction of the rack gear as much as the number of edges of the rack gear.
And, the housing may include a first housing and a second housing installed such that they can communicate with both ends of the motor.
Further, the housing is installed to communicate with one end of the motor at a pinion side and the housing of the motor can be formed integrally with the other end of the motor.